Heat conduction and thermomolecular orientation in diatomic fluids: a non-equilibrium molecular dynamics study

We investigate heat transfer in fluids consisting of diatomic molecules in a wide range of thermodynamic conditions, from densities and temperatures characteristic of the liquid state to supercritical conditions. The interactions are modelled using a two-centre Lennard-Jones model, which enable us to quantify the impact that the incorporation of dispersion interactions has on the recently reported thermo-molecular orientation effect [F. Romer, F. Bresme, J. Muscatello, D. Bedeaux, and J. M. Rubi, Phys. Rev. Lett. 108 (2012), p. 105901]. The temperature gradient imposes a preferred orientation on the molecules. The orientation is stronger in the liquid state and for heteronuclear molecules featuring a large asymmetry in the diameters of the two atoms. We also analyse the microscopic mechanism of heat transport. The transport mechanism is dominated by collisional terms, hence following the general trend observed in liquids. The larger site in the molecule transports a larger amount of energy, the latter being proportional to the site exposed area. We also show that the molecular anisotropy has a large impact on the reduced thermal conductivity of the fluid, being larger for homonuclear molecules. The treatment of the molecules intramolecular bonds, rigid or flexible, does not have a significant impact on the thermal conductivity of the fluid.